1. Field of the Invention
The present invention relates to a reticle with a pattern which is able to mitigate lens heating by redistributing heat onto a lens, and to a method for designing a reticle that redistributes heat onto a lens to mitigate lens heating. In particular, the present invention relates to a reticle with a redistributive pattern which is able to mitigate lens heating without substantially changing the imaging of a target pattern, and a method for designing the redistributive pattern on a reticle to redistribute heat onto a lens to mitigate lens heating.
2. Description of the Prior Art
Lens heating is a phenomenon which is driven by a high power laser that has been absorbed by a lens material or a lens mounting. Lens heating is a common issue in the semiconductor industry, especially for memory manufacturers, and is caused by an unbalanced heat load in the lens due to a highly periodic structure on a reticle.
Once a lens is heated, aberrations will be introduced into the lens system and will degrade the imaging quality. Due to this lens heating problem, the lens aberrations increase when a same pattern is continuously exposed on the scanner. The aberrations can drift and become larger or smaller. In other words, if an aberration starts at some value, it can be positive (go up) or negative (go down) as the lens is heated. The hotter spots may make the aberration drift positive or negative. Lens heating can be reduced if the heat load is more balanced by rearranging the dummy area which is the area that will not have any pattern that can be imaged onto a wafer.
When a reticle only has an entire live pattern, such a method is useless because there is no dummy area to accommodate the solution. A live pattern is the pattern which needs to be transferred and printed onto a wafer. It can be electrically important for device circuits. It can also be some pattern that is not electrically important, such as CMP pads. A dummy area is the area which has no live pattern.
For example, in dummy area, a proposal known as SRF (sub-resolution fill) is used to fill the area with the sub-resolution feature. FIG. 1A illustrates an example of the sub-resolution fill solution. A reticle 10 includes a dummy area 11 and a live pattern area 13. The dummy area 11 is full of sub-resolution fill 12 and the live pattern area 13 full of live pattern 14. When light 50 pass through the reticle 10, a heat load 31 is distributed on the lens 30 and a corresponding image 44 is printed in the wafer 40. The heat load is not a direct copy of what the pattern is originally like. It is the diffraction pattern of the reticle 10. Because the sub-resolution fills 12 are below the resolution limit, the dummy area 11 printed on the wafer 10 will still not show any pattern. The sub-resolution fills 12 of the sub-resolution fill solution are accordingly able to cope with the lens heating problem.
Also, the reticle 10 may include other pattern which is not electrically related or important. FIG. 1B illustrates an example of the CMP (chemical mechanical polishing) fill solution. A reticle 10 includes a CMP pad area 11 and a live pattern area 13. The CMP pad area 11 is full of pad pattern 12 and the live pattern area 13 full of live pattern 14. The pad pattern 12 is used to facilitate the CMP procedure and not electrically related or important. When light 50 pass through the reticle 10, a heat load 31 is formed on the lens 30 and a corresponding image 44 is printed in the wafer 40. Also, the pad pattern image 42 is formed on the wafer 40. Because the pad pattern 12 is used to facilitate the CMP procedure and not electrically related or important, area filled with pad pattern image 42 is not electrically related or important but for CMP benefit. There is no lens heating benefit.
Currently, there is no prior art that utilizes the live pattern area for lens heating mitigation.